The present disclosure generally relates to a pipetting unit and method for pipetting a volume of test liquid and, in particular, to a pipetting unit and method for pipetting a volume of test liquid for carrying out an in vitro diagnostic test such as, for example, a coagulation test.
Different types of pipetting units and pipetting methods have been implemented in various analytical instruments. Among the various pipetting methods, the most challenging ones are those requiring aspirating a volume of test liquid through a cap of a test liquid container. This is typically done by piercing with the nozzle of the pipetting unit a septum of elastomeric material sealing a test liquid container, in which the test liquid is contained. Precision of volume and reproducibility may be particularly hard to achieve when the test liquid container is partially evacuated and/or the volume to be aspirated is small, e.g. below 5-10 microliters. This is due to the fact that air may be present in the pipetting unit and also to the fact that negative pressure, i.e. a pressure lower than atmospheric pressure, is present in the liquid container. This negative pressure, which may be different from container to container, may have an effect on the air in the pipetting unit. This in turn may affect the actual volume of liquid being aspirated and the position of the aspirated volume in the pipetting unit. This means that a first pipetting error may occur when aspirating (a wrong volume) and a second pipetting error may occur when dispensing (wrong position) possibly resulting in only part of the aspirated test liquid being dispensed or no test liquid at all being dispensed.
In order to improve precision of pipetting, the pipetting unit may be operated with a system liquid, in which the presence of air in the pipetting unit is minimized. However a small amount of air, may be still dispersed in the system liquid, e.g. in the form of microbubbles. Air is typically present also at the extremity of the nozzle e.g. due to evaporation of system liquid or on purpose by aspirating a plug of air in order to separate the system liquid from the test liquid to be aspirated. This air, which is affected by the pressure conditions inside the container, may be the major responsible for pipetting errors especially for small volumes as mentioned above.
An additional potential source of errors is the wetting of the outside of the nozzle, i.e. the tip and sides of the nozzle, when aspirating the test liquid. Also, when withdrawing the nozzle through the cap of the container, liquid may be present on the underneath of the cap resulting in loss of aspirating liquid or in additional pick up of liquid, either via the inside or outside of the nozzle.
One way to partly solve this problem is to aspirate an excess of test liquid and discard a first volume, possibly comprising air and test liquid, before dispensing the volume required for the test into a reaction vessel.
Dispensing is normally done by positioning the pipetting nozzle above a waste compartment and above the reaction vessel respectively. The pipetting nozzle is then washed from outside by dipping the nozzle into a wash liquid and from inside by flushing system liquid before aspirating another test liquid. This way of dispensing is also prone to errors because of surface effects at the tip and sides of the nozzle.
Therefore, there is a need for a pipetting unit and method which provide a more reproducible and more precise pipetting of test liquids regardless of the volume required for the test and the type of test liquid container.